Hertzian Fracture, Kinetic Friction and the Mechanical Impact LOX Compatibility Testing of Polymers and Polymer Matrix Composites

Abstract

A cascade of probabilistic mechanisms is proposed to better understand the 72 ft-lbs mechanical impact in liquid oxygen ignition threshold sensitivity testing of polymeric and polymer matrix composite materials. This model teams the well established and understood mechanisms of Hertzian fracture and kinetic friction. In conjunction they explain the mechanical fracturing of the specimen and the very rapid transformation of the test's mechanical energy into temperature rising heat energy. The model also provides the first explanation for the random probabilistic occurrence or nonoccurrence of ignitions in tested materials.

A Review of the Polymer for Cryogenic Application: Methods, Mechanisms and Perspectives

“…The test equipment, parameters and steps were standardized (ASTM D2512, ASTM G86) [9,17,142]. According to ASTM test standard, the specimen of the material is placed at the bottom of an aluminum cup filled with LOX, a flat-tipped steel pin resting on the specimen, and then a 10 Kg plummet drop from 1 m height to strike the steel pin(Figure 12a) [142,[144][145][146][147]. During the test, any explosion, burning, noticeable ash, or charring of specimen is considered as a reaction.…”

“…It has been concluded that it is a strict "pass/fail" test that material must meet if they are to be used in the LOX system. Based on the test standard (ASTM D2512-95), Gerzeski proposed a cascade of probabilistic mechanisms that teams Hertzian fracture and Kinetic friction to study the mechanical impact in LOX ignition threshold sensitivity testing of polymer and polymer composites [144]. A portion of the mechanical impact energy is transformed into thermal energy and a local hot spot formed in the specimen.…”

“…The local temperature of the specimen could be increased by hundreds of degrees Celsius in a short time (1µs or less) due to the thermal energy that transformed from mechanical impact energy [145,146]. During the mechanical impact test, the contract surface condition between specimen and striker and the relative position of specimen and striker may both affect the test result (Figure 12c,d) [144]. Wang et al used a commercial finite element analysis tool to analyze the transient fracture mode and energy conversion process of thermoset polymer during the impact test (ASTM D2512) qualitatively [47].…”

“…The results demonstrated that about 25% of the Kinetic energy (98J) was transformed into sliding energy and the rest was transformed into internal energy. [142,[144][145][146][147]. During the test, any explosion, burning, noticeable ash, or charring of specimen is considered as a reaction.…”